As the efficient encoding method and the efficient encoding apparatus for musical signals, or audio signals such as speech signals, etc., a large number of technologies are known.
For example, as one method thereof, there is a blocking frequency band division system of dividing an audio signal in the time region into blocks every predetermined unit time along the time axis to orthogonally transform signals in the time region every respective blocks into signals in the frequency region to further divide them into signal components in a plurality of frequency bands to carry out re-quantization/encoding thereof every respective frequency bands. This system is generally called Transform Coding.
This method has been already filed as a patent application by the inventors and the applicant of this application, and is disclosed in, e.g., U.S. Pat. Specification No. 5,301,205.
As another method, there is non-blocking frequency band division method of dividing an audio signal in the time region into signal components in plural, e.g., about 20 frequency bands without carrying out blocking every unit time to encode them. In general, method called Sub Band Coding (SBC) is known. For example, such method is disclosed in U.S. Pat. Specification No. 4,896,362 and U.S. Pat. Specification No. 5,105,463.
Further, there are also proposed an efficient encoding method and an efficient encoding apparatus in which the Sub Band Coding and the Transform Coding described above are combined. This system is a method of carrying out band division by the Sub Band Coding thereafter to orthogonally transform signals every respective bands into signals in the frequency region to implement coding to the orthogonally transformed signals every respective bands.
As this method, there is a method disclosed in, e.g., U.S. Pat. Specification No. 4,972,484.
Here, as band division filter of the above-described Sub Band Coding, there is a filter, e.g., Quadrature Mirror Filter (QMF), etc. This filter is described in 1976 R. E. Crochiere Digital coding of speech in subbands Bell Syst. Tech. J. Vol. 55, No. 8 1976.
Moreover, in ICASSP 83, BOSTON Polyphase Quadrature filters A new subband coding technique Joseph H. Rothweiler, method and apparatus for filter division of equal bands are described.
Moreover, as the above-described orthogonal transform method, there is known a method of dividing an input audio signal into blocks every predetermined unit time (frame) to carry out, every blocks, Fast Fourier Transform (FFT), Discrete Cosine Transform (DCT), or Modified Discrete Cosine Transform (MDCT), etc. to thereby transform signals on the time base into signals on the frequency base.
In the embodiment of this application, MDCT is used as orthogonal transform processing. This MDCT is described in ICASSP 1987 Subband/Transform Coding Using Filter Bank Designs Based on Time Domain Aliasing Cancellation J. P. Princen A. B. Bradley Univ. of Surrey Royal Melbourne Inst. of Tech.
Further, as frequency division width in the case of quantizing respective frequency components, it is effective to determine band division width by taking into the hearing sense characteristic of the human being. In actual term, bandwidths such that according as frequency shifts to higher frequency band side, bandwidths become broader, which are called critical bands, are used. Audio signal of 0.about.20 Khz is divided into signals in plural (e.g., 25) bands.
Critical bands refer to frequency bands divided in consideration of the hearing sense characteristic of the human being, and are bands that narrow band noises of the same intensity in the vicinity of frequency of a certain pure sound have when the pure sound is masked by those band noises.
Moreover, in encoding data every respective bands at this time, coding is carried out by bit quantity determined by predetermined or adaptive bit allocation every respective bands. For example, in encoding MDCT coefficient data obtained by the MDCT, coding is carried out by determined bit quantity,
In regard to the bit allocation, the following two literatures are known.
In IEEE Transactions of Acoustics, Speech, and Signal Processing, vol. ASSP-15, No. 4, August 1977, technology for carrying out adaptive bit allocation on the basis of magnitudes of signals every respective bands is described.
Moreover, in ICASSP 1980 the critical band coder--digital encoding of the perceptual requirements of the auditory system M. A. Kransner MIT, there is described technology in which hearing sense masking is utilized to thereby obtain necessary signal-to-noise ratios every respective bands to carry out fixed bit allocation.
Meanwhile, there is limitation in bit quantity per unit time by bit rate determined by recording density in the recording medium, and transmission path capacity or transmission rate (speed) in broadcasting/communication. In view of this, in order to satisfy bit rate, efficient encoding method as previously described is used.
However, the above-described conventional bit allocation technology is technology in which bit allocations every respective channels are respectively independently taken into consideration (hereinafter referred to as Independent Allocation for the brevity). In other words, bit quantities of respective channels are certain fixed quantities. Accordingly, such a bit allocation to bridge over a plurality of channels is not taken into consideration.
On the contrary, the inventors of this application have already proposed, in the specifications and drawings of Japanese patent application Ser. No. 15,492/1993 as the Japanese Patent Application and U.S. patent application Ser. No. 08/184,471 corresponding that application, a method in which common bits which can be used for a plurality of channels are ensured to allocate suitable quantity of bits to channels for which bits are required.
Since bit allocations are not independent every respective channels, such a method is called inter-channel bit allocation (hereinafter referred to as Dependent Allocation for the brevity).
The former proposal of the inventors of this application ensures common bits, thus contributing to improvement in sound quality. However, it can be also said that the method of ensuring common bits at all times is redundant.
Ordinarily, in the case where there are a plurality of channels, differences occur by information quantities of respective channels in bit quantities that respective channels require.
For example, in the case where audio signal is stereo, it is now assumed that bit quantity that the right channel requires is 120% of the reference (standard) quantity, and, on the other hand, bit quantity that the left channel requires is 50%. In such case, bit quantity allowed for the right channel is as far as 100% of the reference quantity. As a result, deterioration in sound quality would take place by 20% of insufficient quantity.
On the contrary, there exist excess bit quantity which is as great as 50% of the reference quantity, and redundant bit quantity of 50% of the reference quantity is used for that left channel as so called over quality. Of course, redundant bits contribute to improvement in sound quality, but this improvement is nothing but improvement to such a degree which it is not perceived by the hearing sense of the human being.
Meanwhile, in this example, it is conceivable to use bit quantity of 50% of the reference quantity which was redundant at the left channel for 20% which is insufficient at the right channel (hereinafter referred to as Subsidiary Allocation for the brevity).
If such a method can be realized, it is possible to satisfy a predetermined bit rate while maintaining high sound quality at both channels.
Particularly, in the case of audio signals of music or cinema, etc., unlike telephone, there are many instances where one information is constituted by sets of a plurality of channels. Accordingly, there are instances where redundant bits might take place at any channel or channels of a plurality of channels. It is therefore considered that dependent allocation is effective.
Meanwhile, in this case, another problem takes place. Namely, conventional decoder (efficient decoding apparatus) decodes signals of a recording medium on which encoded signals based on independent allocation are recorded.
For this reason, it is impossible to decode, with conventional decoder (decoding apparatus), signals of a recording medium on which signals encoded by dependent allocation are recorded.
In addition, if decoding apparatus for decoding signals of a recording medium on which encoded signals are recorded by dependent allocation cannot decode signals of recording media already on the market, i.e., recording media on which signals encoded only by independent allocation are recorded, this is considerably disadvantageous to user.